ALBERT

Description: A satellite-based global analysis of high-resolution (0.25o) ocean surface turbulent latent and sensible heat fluxes was developed by the Objectively Analyzed air-sea Fluxes (OAFlux) project. Resolving air-sea flux down to the order to 0.25o is critical for the description of the air-sea interaction on mesoscale scales. In this study, we evaluate the high-resolution product in depicting air-sea exchange in the eddy-rich Gulf Stream region. Two approaches were used for evaluation, one is point-to-point validation based on six moored buoys in the region, and another is basin-scale analysis in terms of wavenumber spectra and probability density functions (PDFs). An intercomparison is also carried out between OAFlux-0.25o, OAFlux-1o, and four atmospheric reanalyses. Results indicate that OAFlux-0.25 o is able to depict sharp oceanic fronts and has the best performance among the six participating products in comparison with buoy measurements. The mean OAFlux-0.25 o differences in latent and sensible heat flux with respect to the buoy are 7.6 Wm -2 (7.7%) with root-mean-square (RMS) difference of 44.9 Wm -2 , and 0.0 Wm -2 with RMS difference of 19.4 Wm -2 , respectively. Large differences are primarily due to mismatch in SST between gridded data and point measurements when strong spatial gradients are presented. The wavenumber spectra and decorrelation length scale analysis indicate OAFlux-0.25 o depicts eddy variability much better than OAFlux-1 o and the four reanalyses; however, its capability in detecting eddies with smaller scale still needs to be improved. Among the four reanalyses, CFSR stands out as the best in comparison with OAFlux-0.25°.

Description: Breaking waves and Langmuir circulation are two important turbulent processes in the wind-driven upper ocean. To investigate their roles in generating turbulence in the surface boundary layer of a coastal ocean, a Large Eddy Simulation (LES) model is used to simulate the turbulence measurements collected at the Marthad's Vineyard Coastal Observatoryd's Air-Sea Interaction Tower, during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) experiment in 2003. The model provides reasonable predictions for the vertical profiles of vertical velocity variance, turbulent kinetic energy (TKE), energy dissipation rates and heat flux. It shows breaking waves dominating turbulence generation near the ocean surface and turbulent large eddies characteristic of Langmuir circulation deeper in the water column. Diagnostic analysis of TKE budget in the model shows a dominant balance between turbulent transport and dissipation near the surface and a dominant balance between shear production and dissipation at deeper depths. Although the Stokes production is a significant term in the TKE budget balance near the surface, it is smaller than shear production. The turbulent transport is large in the near-surface zone and is still significant in the region affected by Langmuir circulation. These results are in agreement with a conclusion inferred from a recent analysis of the near-surface turbulence measurements at the CBLAST site.

Description: [1]  We investigated the evolution of seismicity and deformation in the unstable eastern flank of Etna volcano over a thirty-year period (from 1980 to 2012). A significant temporal correlation has been revealed between periods of flank acceleration and intensified seismic activity by comparing seismicity along the northern border (Pernicana fault system) of the sliding flank and the deformation of the eastern flank. Two marked phases have been observed in 1984-1986 and in the years following 2002. These two phases are separated by an intermediate phase from 1987 to 2001, in which the flank sliding slowed down and the seismicity dropped drastically. This common temporal evolution in the deformation rate and seismic release supports the hypothesis that the seismicity in the northern border can be viewed as a marker of the response to accommodate the stress exerted by the traction of the eastern flank sliding. This interplay has also been corroborated by Finite Element Method (FEM) numerical computations that highlight a good correlation between the seismicity pattern and areas of positive stress changes induced by the sliding surface. The two intense phases of flank acceleration are representative of two main different sources: volcano flank instability stretching the eastern sector in the first 1984-1986 phase and magmatic intrusions pushing the eastern flank seaward since the 2002-03 eruption. Establishing the relationship between flank acceleration and seismic activation, therefore, contributes to understanding Etna's mechanical behavior, and provides insights into the processes regulating the unstable flank response.

Description: [1]  Numerous studies in the Central Pyrenees have provided evidence for a rapid phase of exhumation of this mountain belt during the Late Eocene (37–30 Ma). Simultaneously, the closure of the Ebro foreland basin allowed the accumulation of sediments at the southern Piedmont, which partially covered the fold-and-thrust belt from Late Eocene ( e . g . when it was still actively deforming) to Miocene times. We aim here at understanding the consequences of such syn-tectonic sedimentation on the Southern Pyrenean fold-and-thrust belt by using a 2-D numerical model that reproduces the development of a thin-skinned wedge subject to different modes of sedimentation and erosion. The results show contrasting fold-and-thrust belt behavior when applying aggrading or prograding sedimentation, which we link to the critical state of the wedge. When the sediments are sourced from the hinterland (progradation), the thrusting propagates toward the foreland; whereas when the sediments aggrade from the basin, the thrusting sequence migrates backward. This latter mode shows patterns of deformation that compare favorably to the Pyrenean thrusting sequence observed during Eocene-Miocene times.

Description: [1]  In southern California, fault slip rate estimates along the San Andreas fault (SAF) and Garlock fault from geodetically-constrained kinematic models are systematically at the low end or lower than geologic slip rate estimates. The sum of geodetic model slip rates across the Eastern California Shear Zone is higher than the geologic sum. However, the ranges of reported model and geologic slip rate estimates in the literature are sufficiently large that it remains unclear whether these apparent discrepancies are real, or attributable to epistemic uncertainties in the two types of estimates. We further examine uncertainties in geodetically-derived slip rate estimates on major faults in southern California by conducting a suite of inversions with four kinematic models. Long-term-rigid elastic block models constrained by the geologic slip rates cannot fit the present-day GPS-derived velocity field. Deforming (permanent off-fault strain) elastic block models and viscoelastic earthquake cycle block models constrained by geologic slip rates can fit the present-day GPS-derived velocity field with 28-33% of the total geodetic moment rate occurring as distributed deformation off of the major faults. Models incorporating viscoelastic mantle flow predict systematically higher slip rates than purely elastic models on many of the the major southern California faults with ranges of (elastic/viscoelastic) 29-34/30-37 mm/yr for the Carrizo SAF segment, 20-24/20-32 mm/yr for the Mojave SAF segment, 14-17/18-22 mm/yr for the Coachella SAF segment, 13-19/14-22 mm/yr for the San Jacinto fault, and 5-11/5-11 mm/yr for the western Garlock fault.

Description: This paper proposes a method to identify blocking onset and decay by means of two stability indicators: enstrophy advection and its integral. The key to this technique is the use of local Lyapunov exponents for the barotropic vorticity equation which can be approximated by the integral of enstrophy (IRE) over a fixed, finite, region. The IRE can then be viewed as a measure of stability. However, by differentiating the IRE with respect to time, two measures of stability can be derived to assess blocking onset and decay: (i) the integral of enstrophy advection (DIRE), for which a time series is used to assess stability; and (ii) enstrophy advection, for which contours are plotted in conjunction with 500 hPa heights to locate blocking. One year of Northern Hemisphere blocking events from July 2011-July 2012 are studied to demonstrate that the integral of enstrophy advection is a useful diagnostic. In particular, time series of IRE and DIRE for four of the blocking cases are presented, while contour plots of enstrophy advection for one case are presented. In all cases studied, the diagnostics were seen to detect the instability in an incipient blocking event and in its decay.

Description: A Gulf of Mexico performance evaluation and comparison of coastal circulation and wave models was executed through harmonic analyses of tidal simulations, hindcasts of Hurricane Ike (2008) and Rita (2005), and a benchmarking study. Three unstructured coastal circulation models (ADCIRC, FVCOM, and SELFE) validated with similar skill on a new common Gulf scale mesh (ULLR) with identical frictional parameterization and forcing for the tidal validation and hurricane hindcasts. Coupled circulation and wave models, SWAN+ADCIRC and WWMII+SELFE, along with FVCOM loosely coupled with SWAN, also validated with similar skill. NOAA's official operational forecast storm surge model (SLOSH) was implemented on local and Gulf scale meshes with the same wind stress and pressure forcing used by the unstructured models for hindcasts of Ike and Rita. SLOSH's local meshes failed to capture regional processes such as Ike's forerunner and the results from the Gulf scale mesh further suggest shortcomings may be due to a combination of poor mesh resolution, missing internal physics such as tides and nonlinear advection, and SLOSH's internal frictional parameterization. In addition, these models were benchmarked to assess and compare execution speed and scalability for a prototypical operational simulation. It was apparent that a higher number of computational cores are needed for the unstructured models to meet similar operational implementation requirements to SLOSH, and that some of them could benefit from improved parallelization and faster execution speed.

Description: Pairs of moorings equipped with current profilers were deployed at each end of the Dardanelles Strait and remained in place for over 13 months. Current observations were able to resolve well the exchange flow and volume fluxes. Volume fluxes showed distinct temporal variability in upper and lower layers, especially evident on synoptic time scales. The synoptic flux variability in the upper layer was coherent with the local atmospheric forcing and the bottom pressure anomaly gradient, while the flux variations in the lower layer were related to the bottom pressure anomaly gradient. Estimated volume flux values were often two or more times larger than their respective annual means. Annual upper-layer flux means were 25.66·10 -3 and 36.68·10 -3 Sv, whereas the lower-layer averages were 14.02·10 -3 and 31.67·10 -3 Sv for the Marmara and Aegean sections, respectively. The fluxes also showed that there was a net low-salinity water outflow to the Aegean Sea, and that they varied weakly on longer time scales (monthly to seasonal). High-salinity water fluxes (≥ 39 psu) were used to calculate strait-averaged vertical eddy diffusivities which ranged between 10 -4 and 10 -2 m 2 s -1 . Additionally, microstructure observations were used to evaluate vertical eddy diffusivities. These estimates indicated that mixing in the strait varied spatially and temporarily, and it was dependent on complex strait geometry, exchange flow status, and partially on meteorological conditions. Large values of eddy diffusivities, with a depth-averaged mean of 1.3·10 -2 m 2 s -1 , and vigorous mixing were found in the Nara Pass, the narrowest section in the Dardanelles Strait.

Description: In this paper, simulated variability of the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Overturning Circulation (AMOC) and their relationship have been investigated. For the first time, climate models of the Coupled Model Intercomparison Project phase 5 (CMIP5) provided to the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC-AR5) in historical simulations have been used for this purpose. The models show the most energetic variability on the multidecadal timescale band both with respect to the AMO and AMOC, but with a large model spread in both amplitude and frequency. The relationship between the AMO and AMOC in most of the models resembles the delayed advective oscillation proposed for the AMOC on multidecadal timescales. A speed up (slow down) of the AMOC is in favor of generating a warm (cold) phase of the AMO by the anomalous northward (southward) heat transport in the upper ocean, which reversely leads to a weakening (strengthening) of the AMOC through changes in the meridional density gradient after a delayed time of ocean adjustment. This suggests that on multidecadal timescales the AMO and AMOC are related and interact with each other.

Description: Atmospheric boundary layer rolls and their impact on upper ocean circulation were investigated using a combination of two high-resolution data sources: (1) data from the Advanced Synthetic Aperture Radar (ASAR) onboard ENVISAT with a spatial sampling of approximately 500m×500m, and (2) continuous observations taken at the research platform FINO 1 with two-minutes temporal sampling at eight heights between 33 and 100m. The parallel analysis of instantaneous image data in combination with the FINO 1 time series enabled us to quantify both the spatial and temporal dynamics of mesoscale and submesoscale wind variations. The influence of these variations with different temporal and spatial scales on the hydrodynamics of the German Bight was addressed using outputs from a three-dimensional circulation model. It was demonstrated that while the coupling between wind and tidal forcing triggered substantial responses at mesoscales, the response of surface currents and sea surface temperature to the atmospheric boundary layer rolls appeared relatively weak. However, these ocean surface responses closely follow the surface footprint of the atmospheric boundary layer rolls, the signatures of which become more pronounced in the absence of strong tidal flows.

Description: State estimation techniques have been well established in open ocean systems; however they are less often used in coastal applications due to non-linearity. Using 4D-variational data assimilation in a triple one-way nested system, we investigate the processes that control coastal dynamics for a test case along the western coast of Oahu, Hawaii. All available observations are combined with the model dynamics for 13 months. Over this time, the residual error between the model and observations was improved by nearly 30% in the surface temperature and 34% in the along-shore ADCP currents. The barotropic and baroclinic tides are found dominate the local circulation; however, island and atmospheric interaction generates an island wake effect that is important to the sub-tidal dynamics of the region. The baroclinic tides exhibit well defined energy paths, and the initial condition corrections, despite altering the density waveguide, have little influence on the propagation of the baroclinic energy, which is controlled by the propagation of baroclinic tides generated outside of the domain. We find the larger-scale, advected dynamics control the local surface circulation through boundary condition adjustment, accounting for 45% of the total corrections made via data assimilation system. The initial conditions controls little of the evolution of this local, coastal flow and has a short persistence. The wind-stress control vector is important in the central region of the domain inducing flow toward the lee of the island. Our results reveal that coastal studies may not be initial value problems, rather they are forced problems that require a knowledge of the large-scale energy propagated into the region.

Description: The effects of solar radiation diurnal cycle on intraseasonal mixed layer variability in the tropical Indian Ocean during boreal wintertime Madden-Julian Oscillation (MJO) events are examined using the HYbrid Coordinate Ocean Model. Two parallel experiments, the main run and the experimental run, are performed for the period of 2005-2011 with daily atmospheric forcing except that an idealized hourly shortwave radiation diurnal cycle is included in the main run. The results show that the diurnal cycle of solar radiation generally warms the Indian Ocean sea surface temperature (SST) north of 10°S, particularly during the calm phase of the MJO when sea surface wind is weak, mixed layer is thin, and the SST diurnal cycle amplitude ( dSST ) is large. The diurnal cycle enhances the MJO-forced intraseasonal SST variability by about 20% in key regions like the Seychelles–Chagos Thermocline Ridge (SCTR; 55°-70°E, 12°-4°S) and the central equatorial Indian Ocean (CEIO; 65°-95°E, 3°S-3°N) primarily through nonlinear rectification. The model also well reproduced the upper-ocean variations monitored by the CINDY/DYNAMO field campaign between September-November 2011. During this period, dSST reaches 0.7°C in the CEIO region, and intraseasonal SST variability is significantly amplified. In the SCTR region where mean easterly winds are strong during this period, diurnal SST variation and its impact on intraseasonal ocean variability are much weaker. In both regions, the diurnal cycle also has large impact on the upward surface turbulent heat flux Q T and induces diurnal variation of Q T with a peak-to-peak difference of O (10 W m -2 ).

Description: The horizontal and vertical flux of particulate material in the nearshore of southern Lake Michigan (0 – 40 m) was estimated with the naturally occurring radionuclide 234 Th. Horizontal fluxes of 234 Th supplemented apparent vertical fluxes of 234 Th in the water column (based on local 234 Th/ 238 U disequilibria) by a factor of 7 to 14, reinforcing the importance of lateral transport in coastal environments. Calculated onshore transport of particulate material across the 40 m isobath was as high as 1.1 × 10 6 kg km -1 d -1 , and exceeds estimates of terrigenous (riverine and bluff erosion) loading. Estimates of onshore flux of organic carbon exceeded areal primary productivity by as much as ~ 300 %, and should be considered in nearshore carbon budgets. Bottom tethered sediment traps (placed 5 m above the bottom) measured sedimentation rates that were approximately 1 order of magnitude lower than 234 Th derived mass fluxes from the water column and ~ 2 orders of magnitude lower than 234 Th derived mass fluxes to the lakebed. We ascribe this difference to under-collecting by the sediment trap either because of trap hydrodynamics or flux occurring below the trap capture plane. Cross-shore fluxes showed a periodicity of ~ 4 days and correlated strongly with a topographic vorticity wave that is present throughout the year in southern Lake Michigan. The impact of this wave (as a driver of bidirectional cross-shore flux) on biogeochemical cycling and both nearshore and offshore food webs has not yet been explicitly considered.

Description: Time series (1990-2011) of sea ice thickness observed by moored sonars in the Transpolar Drift in Fram Strait are examined. Contrasting the post 2007 years against the 1990s, three remarkable changes in the monthly ice thickness distributions are highlighted: 1) The thickness of old level ice (modal thickness) is reduced by 32 percent, 2) the old ice modal peak width is reduced by 25 percent, and 3) the fraction of (ridged) ice thicker than 5 m is reduced by 50 percent. The combined effect on the mean ice thickness is a reduction from an annual average of 3.0 m during the 1990s to 2.2 m during 2008-2011. Most of the thinning took place after 2005-2006. While the old ice modal thickness and peak width show signs of recovery after 2008, the decreasing trend in fraction of ridged ice and mean ice thickness persists until the end of the record in 2011. The ice observed in Fram Strait carries an integrated signal of Arctic change due to the advection of ice from many sites in the Arctic. Based on concurrence in timing, we conclude that much of the thinning quantified here is reflecting recent change in the age composition of the Arctic ice cover towards younger ice. The old level ice remains thin, and as such the ice cover remains preconditioned for new summers of very low sea ice extent.

Description: Hydrographic observations, ocean state estimates, and ocean objective analyses are combined to investigate the decadal variations of the North Equatorial Current (NEC) in the Pacific at 137°E during the last three decades (1975-2005). Observations show that the decadal NEC transport has three maxima around 1980/1981, 1994/1995, and 2004/2005 and two minima around 1989/1990 and 1999/2000, respectively. Associated with these maxima/minima, the sea surface height (SSH) falls/rises and the subsurface isopycnals shoal/deepen in the southern part of NEC, resulting in westward/eastward zonal velocity anomalies. Results from the ocean state estimates and ocean objective analyses show good agreement with observations. Further analysis indicates that the observed zonal velocity anomalies at 137°E are part of the cyclonic/anticyclonic gyre anomalies formed in the tropical northwestern Pacific east of the Philippines, coinciding with the tropical gyre. Results from a 1½ layer reduced gravity model suggest that these oceanic variations are mainly controlled by the decadal wind forcing in the tropical western Pacific and can be attributed to both local Ekman dynamics and baroclinic Rossby wave propagation.

Description: Based on observations and ocean reanalysis, this study analyzes the variability of salinity and its related ocean dynamics in the equatorial Indian Ocean (IO). The results show significant interannual variability of salinity associated with the Indian Ocean Dipole (IOD) mode in the boreal fall. During the positive phase of IOD (pIOD), when anomalous easterly winds prevail, westward advection along the equator strengthens in summer, while the eastward advection associated with the Yoshida-Wyrtki Jet weakens in fall. Analysis of salinity budget indicates that salinity anomalies are mainly due to advection, of which zonal component is dominant. As zonal current anomalies are symmetric with respect to the equator, the equatorward large northern IO zonal salinity gradient is more important than the current anomalies in determining the asymmetric distribution of low-salinity advection. During the mature phase of pIOD, low-salinity water is advected westward, which in turn shoals the surface mixed layer, thereby providing a favorable condition for warmer sea-surface temperature in the western equatorial IO. During the decay phase of pIOD, low-salinity water is advected across the equator to the southwestern IO. When pIOD concurs along with El Niño, the strengthened off-equatorial anticyclonic circulations, which is associated with El Niño, advect low-salinity water poleward after the decay phase.

Description: The Weather Research and Forecasting (WRF) model was used to simulate the evolution of Tropical Storm Ivan (2004) in the southeast (SE) US using both the Yonsei University (YSU) and Mellor-Yamada-Janjić (MYJ) boundary layer parameterizations. In contrast with tropical cyclone (TC) simulations over the ocean, the effect of surface layer becomes secondary for a dissipating hurricane along its terrestrial track. Although these two schemes can reproduce Ivan reasonably well, our results suggest that the mixing properties for damped mechanical turbulent conditions (weakly stable) are strongly underestimated by both parameterizations. This underestimation impacts the thermodynamic properties of the storm, leading to significant differences in the storm areal extent and the simulated precipitation fields. Suggestions for further improvements are provided. An evaluation of the impact of using or not using a convective parameterization, specifically the Kain-Fritsch (KF) scheme, at 3 km grid spacing shows marginal impact on storm coverage, intensity and precipitation, except for the presence of widespread light rainfall in the Piedmont east of the mountains when the KF is employed. Analysis of the thermal structure of the simulated storm indicates that, in the inner-storm region, the KF is either not activated or primarily produces ( parameterized ) shallow convection. As a result, the net heating tendency associated with adiabatic and diabatic processes is almost unaltered inside the storm, together with a nearly equivalent surface momentum sink, leading to similar storm areal extent and intensity. Light rainfall to the east of the mountains can be due to the trigger mechanism of KF, which depends on boundary layer convergence, forcing parameterized deep convection near the coast, where surface roughness changes enhance convergence.

Description: Heavy precipitation events (HPEs) affect the south-eastern area of France frequently during the months of September to November. Very high amounts of rain can fall during these events, with the ensuing flash-floods causing widespread damage. The cases of the 6th of September 2010 and the 1st to the 4th of November 2011 represent the different large-scale conditions in which these episodes can occur. These HPEs are forecast with differing levels of skill by the Méso-NH model at a 2.5 km resolution. The case of the 6th of September is used to test different methods of addressing cloud physics parameterisation uncertainties. Three ensembles are constructed, where the warm process microphysical time tendencies are perturbed by different methods. Results are compared by examining the spatio-temporal distribution of the precipitation field as well as looking at ensemble statistics. The ensemble methodology which induces the most dispersion in the rainfall field is deemed the most suitable. This method is then used to examine the sensitivity of four cases from November 2011 to errors in the microphysical and turbulent parameterisations. It appears that according to the model skill for the HPE, the sensitivity to microphysical perturbations varies. Events where the model skill is high (low) show low (moderate) sensitivity. These cases show a stronger sensitivity to perturbations performed upon the turbulent tendencies, while perturbing the microphysical and turbulent tendencies together produces even further dispersion. The results show the importance and the usefulness of ensembles with perturbed physical parameterisations in the forecasting of HPEs.

Description: This paper analyses the annual mean vertical and latitudinal structure of the Brewer-Dobson circulation in the CMIP5 models. The strength of the tropical mass upwelling is found to increase at all altitudes throughout the stratosphere due to climate change. However, the width of the tropical upwelling region narrows below about 20hPa, and widens above 20hPa, suggesting different physical mechanisms may play a role in this change above and below 20hPa. In the lower stratosphere, an equatorward shift in the stationary wave critical line allows waves to propagate further into the tropics. However, in the upper stratosphere, where the behaviour is dominated by what happens during the winter, an increase in the extratropical zonal mean westerly jet leads to a reduced equatorward refraction of planetary waves. The seasonal cycle of the change in the Brewer-Dobson circulation is also considered, and differences are found in the latitudinal structure of the increased extratropical downwelling between the Northern and Southern Hemispheres in winter.

Description: Sub-seasonal forecasts have been routinely produced at ECMWF since 2002 with re-forecasts produced "on the fly" to calibrate the real-time sub-seasonal forecasts. In this study, the skill of the re-forecasts from April 2002 to March 2012 and covering a common set of years (1995 to 2001) has been evaluated. Results indicate that the skill of the ECMWF re-forecasts to predict the Madden Julian Oscillation has improved significantly since 2002, with an average gain of about 1 day of prediction skill per year. The amplitude of the MJO has also become more realistic, although the model still tends to produce MJOs which are weaker than in the ECMWF re-analysis. As a consequence, the ability of the ECMWF model to simulate realistic MJO teleconnections over the northern and southern Extratropics has improved dramatically over the 10-year period. Forecast skill scores have also improved in the Extratropics. For instance, weekly mean forecasts of the North Atlantic Oscillation Index are more skillful in recent years than ten years ago. A large part of this improvement seems to be linked to the improvements in the representation of the Madden Julian Oscillation. Skill to predict 2-metre temperature anomalies over the northern Extratropics has also improved almost continuously since 2002. Changes in the horizontal and vertical resolutions of the atmospheric model had only a small impact on the skill scores, suggesting that most of the improvements in the ECMWF sub-seasonal forecasts were due to changes in model physics which were primarily designed to improve the model climate and medium-range forecasts. The impact of changes in the data assimilation system and in the observing data has not been considered in this study, since all the re-forecasts used for this study were initialized from the same re-analysis over a common set of years.

Description: Dual-polarisation radar measurements provide valuable information about the shapes and orientations of atmospheric ice particles. For quantitative interpretation of these data in the Rayleigh regime, common practice is to approximate the true ice crystal shape with that of a spheroid. Calculations using the discrete dipole approximation for a wide range of crystal aspect ratios demonstrate that approximating hexagonal plates as spheroids leads to significant errors in the predicted differential reflectivity, by as much as 1.5dB. An empirical modification of the shape factors in Gans's spheroid theory was made using the numerical data. The resulting simple expressions, like Gans's theory, can be applied to crystals in any desired orientation, illuminated by an arbitrarily polarised wave, but are much more accurate for hexagonal particles. Calculations of the scattering from more complex branched and dendritic crystals indicate that these may be accurately modelled using the new expression, but with a reduced permittivity dependent on the volume of ice relative to an enclosing hexagonal prism.

Description: The parameterisation of diabatic processes in numerical models is critical for the accuracy of weather forecasts and for climate projections. A novel approach to the evaluation of these processes in models is introduced in this contribution. The approach combines a suite of on-line tracer diagnostics with off-line trajectory calculations. Each tracer tracks accumulative changes in potential temperature associated with a particular parameterised diabatic process in the model. A comparison of tracers therefore allows the identification of the most active diabatic processes and their downstream impacts. The tracers are combined with trajectories computed using model-resolved winds, allowing the various diabatic contributions to be tracked back to their time and location of occurrence. We have used this approach to investigate diabatic processes within a simulated extratropical cyclone. We focus on the warm conveyor belt, in which the dominant diabatic contributions come from large-scale latent heating and parameterised convection. By contrasting two simulations, one with standard convection parameterisation settings and another with reduced parameterised convection, the effects of parameterised convection on the structure of the cyclone have been determined. Under reduced parameterised convection conditions, the large-scale latent heating is forced to release convective instability that would otherwise have been released by the convection parameterisation. Although the spatial distribution of precipitation depends on the details of the split between parameterised convection and large-scale latent heating, the total precipitation amount associated with the cyclone remains largely unchanged. For reduced parameterised convection, a more rapid and stronger latent heating episode takes place as air ascends within the warm conveyor belt.

Description: Motivated by observations of the mean state of tropical precipitable water (PW), a moist, first baroclinic mode, shallow water system on an equatorial β -plane with a background saturation profile that depends on latitude and longitude is studied. In the presence of a latitudinal moisture gradient, linear analysis of the non-rotating problem reveals large-scale, symmetric, eastward and westward propagating unstable modes. The introduction of a zonal moisture gradient breaks the east–west symmetry of the unstable modes. The effects of rotation are then included by numerically solving the resulting eigenvalue problem on an equatorial β -plane. With a purely meridional moisture gradient, the system supports large-scale, low-frequency, eastward and westward moving neutral modes. Some of the similarities, and some of the discrepancies of these modes with intraseasonal tropical waves are pointed out. Finally, a zonal moisture gradient in the presence of rotation renders some of the aforementioned neutral modes unstable. In particular, as per observations of large-scale, low-frequency tropical variability, it is seen that regions where the background saturation profile increases (decreases) to the east favour eastward (westward) moving moist modes.

Description: In this study changes in the Northern Hemisphere winter storm tracks during the 20th century are investigated based on the individual 56 ensemble-members of the 20th Century Reanalysis dataset. It is found that the 20th century trends in storm track activities exhibit large discrepancies between the upper and lower troposphere. In the upper troposphere, a substantial intensification is identified at the poleward and downstream regions of the North Pacific and North Atlantic storm track activities, indicating a large northeastward expansion of storm tracks in the late 20th century. However, in the lower troposphere the synoptic eddy activities, especially in terms of the eddy kinetic energy (EKE) and meridional eddy heat flux, tend to be significantly weakened over the high-latitudes of central-western North Pacific and the upstream regions of the North Atlantic storm tracks. Further inspections find that such strengthening (weakening) of storm tracks in the upper (lower) troposphere are mainly attributed to the increase (decrease) of the baroclinic instability, which is predominantly determined by the meridional temperature gradient changes. Moreover, from a local energetic perspective, the baroclinic generation and barotropic damping of the synoptic eddies are found to be substantially enhanced at the upstream and downstream regions of the two storm tracks in the upper troposphere, respectively, while in the lower troposphere the baroclinic energy conversion to eddies are generally decreased.

Description: We derive a family of ideal (nondissipative) 3D sound-proof fluid models that includes both the Lipps-Hemler anelastic approximation (AA) and the Durran pseudo-incompressible approximation (PIA). This family of models arises in the Euler-Poincaré framework involving a constrained Hamilton's principle expressed in the Eulerian fluid description. The derivation in this framework establishes the following properties of each member of the entire family: the Kelvin-Noether circulation theorem, conservation of potential vorticity on fluid parcels, a Lie-Poisson Hamiltonian formulation possessing conserved Casimirs, a conserved domain integrated energy and an associated variational principle satisfied by the equilibrium solutions. Having set the stage with the derivations of 3D models using the constrained Hamilton's principle, we then derive the corresponding 2D vertical slice models for these sound-proof theories.

Description: [1]  Determining the scale-length, magnitude, and distribution of heterogeneity in the lowermost mantle is crucial to understanding whole mantle dynamics, and yet it remains a much debated and ongoing challenge in geophysics. Common shortcomings of current seismically-derived lowermost mantle models are incomplete raypath coverage, arbitrary model parameterization, inaccurate uncertainty estimates, and an ad hoc definition of the misfit function in the optimization framework. In response, we present a new approach to global tomography. Apart from improving the existing raypath coverage using only high quality cross-correlated waveforms, the problem is addressed within a Bayesian framework where explicit regularization of model parameters is notrequired. We obtain high resolution images, complete with uncertainty estimates, of the lowermost mantle P-wave velocity structure using a hand-picked dataset of PKPab-df, PKPbc-df, and PcP-P differential traveltimes. Most importantly, our results demonstrate that the root mean square of the P-wave velocity variations in the lowermost mantle is approximately 0.87%, which is three times larger than previous global-scale estimates.

Description: Submarine canyons that cut into the continental shelf are regions of enhanced upwelling. The depth of upwelling and flux through the canyons determines their role in exchange between the shelf and the open ocean. Scaling analyses that relate these quantities to the strength of the flow, stratification, Coriolis parameter and topographic shape parameters allow their estimation in the absence of a full numerical simulation or a detailed field study. Here we add the effect of the continental shelf slope to the scaling of the depth of upwelling, upwelling flux, and deep water stretching. The scaling is then tested using a three-dimensional primitive equation model over 18 distinct geometries. The impact of the continental shelf is significant for real canyons with changes in the depth of upwelling of up to 11% and of the flux of upwelling of up to 70%. The numerical simulations clearly show three types of canyon upwelling, a symmetric time-dependent flux, the dominant advection-driven flux and a new flux that appears to be related to internal waves. They also suggest that the canyon width is more important than the upstream canyon shape in determining the strength of the flow across the canyon.

Description: [1]  The Dzhungarian strike-slip fault of Central Asia is one of a series of long, NW-SE right-lateral strike-slip faults that are characteristic of the northern Tien Shan region, and extends over 300 km from the high mountains into the Kazakh Platform. Our field-based and satellite observations reveal that the Dzhungarian fault can be characterised by three 100 km long sections based on variation in strike direction. Through morphological analysis of offset streams and alluvial fans, and through OSLdating, we find that the Dzhungarian fault has a minimum average late Quaternary slip rate of 2.2 ± 0.8 mm/yr and accommodates N-S shortening related to the India-Eurasia collision. This shortening may also be partly accommodated by counter-clockwise rotation about a vertical axis. Evidence for a possible paleo-earthquake rupture indicates that earthquakes up to at least Mw 7 can be associated with just the partitioned component of reverse slip on segments of the central section of the fault up to 30 km long. An event rupturing longer sections of the Dzhungarian fault has the potential to generate greater magnitude earthquakes ( Mw 8), however long time periods (e.g. thousands of years) are expected in order to accumulate enough strain to generate such earthquakes.

Description: The surface wind response to SST and SST meridional gradient is investigated in the Gulf of Guinea by using daily observations and reanalyses in the 2000–2009 decade, with a focus on boreal spring and summer months (May to August), where quasi-biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi-biweekly equatorial sea surface temperature (SST) anomalies. Following a large-scale wind acceleration (deceleration), an equatorial SST cold (warm) anomaly is created within a few days. In order to explain the local atmospheric response to this SST anomaly, the two following mechanisms are invoked: first, a colder (warmer) ocean decreases (increases) the vertical stability in the marine atmospheric boundary layer, which favors a weaker (stronger) surface wind; and second, a negative (positive) anomaly of SST meridional gradient induces a positive (negative) anomaly of sea level pressure meridional gradient, which decelerate (accelerate) the surface wind. The first mechanism has an immediate effect in the equatorial belt between 1°S-1°N (and to a lesser extent between 3°S and 1°S), while the second takes one or two days to adjust and damps anomalous southeasterlies up to 800 hPa in the low troposphere between 7°S and 1°N, through reversed anomalies of meridional SST and pressure gradient. This negative feedback leads to weaker (stronger) winds in the southeastern Tropical Atlantic, which forces the opposite phase of the oscillation within about a week. Around the equator, where the amplitude of the oscillation is found maximal, both mechanisms combine to maximize the wind response to the front fluctuations. Between the equator and the coast, a low-level secondary atmospheric circulation takes control of the surface wind acceleration or deceleration around 3°N, which reduces the influence of the SST front fluctuations.

Description: The purpose of the operational-oriented system COALITION ( C ontext and Scale Oriented Thunderstorm Satellite Predictors Development) is to automatically detect severe thunderstorms early in their development and consequently help weather forecasters to increase lead time when issuing severe weather warnings. This new object-oriented system integrates data provided by different sources. Data from Meteosat Second Generation Rapid Scan Service, weather radar and numerical weather prediction, as well as climatology are utilized by the system. One of its primary purposes is to use all the best operationally available information about convective processes and to integrate it into a heuristic model. Furthermore the orographic forcing, which is often neglected in heuristic nowcasting models, is taken into account and included in the system as an additional convective triggering mechanism. This is particularly important for areas characterized by complex orography like the Alpine region. The COALITION algorithm merges evolving thunderstorm properties with selected predictors. The forecasted evolution of the storm is the result of the interaction between convective signatures and surrounding storm environment. Eight different "object-environment" interactions are analysed in eight modules, providing ensemble nowcasts of thunderstorm attributes (satellite- and radar-based) for the following 60 minutes. All ensemble nowcasts are then combined through a weighting and thresholding scheme and the results are summarized into a single graphical map in order to facilitate user interpretation. The COALITION nowcast system has an update frequency of 5 minutes. The output highlights the cells having a high probability of severe thunderstorm development within the next 30 minutes. Verification statistics confirm that COALITION is able to nowcast the intensity of developing convective cells with sufficient skill up to a lead time of about 20 minutes.

Description: In late February 2010 the extraordinary windstorm Xynthia crossed over Southwestern and Central Europe and caused severe damage, affecting particularly the Spanish and French Atlantic coasts. The storm was embedded in uncommon large-scale atmospheric and boundary conditions prior to and during its development, namely enhanced sea surface temperatures (SST) within the low-level entrainment zone of air masses, an unusual southerly position of the polar jet stream, and a remarkable split jet structure in the upper troposphere. To analyse the processes that led to the rapid intensification of this exceptional storm originating close to the subtropics (30°N), the sensitivity of the cyclone intensification to latent heat release is determined using the regional climate model COSMO-CLM forced with ERA-Interim data. A control simulation with observed SST shows that moist and warm air masses originating from the subtropical North Atlantic were involved in the cyclogenesis process and led to the formation of a vertical tower with high values of potential vorticity (PV). Sensitivity studies with reduced SST or increased laminar boundary roughness for heat led to reduced surface latent heat fluxes. This induced both a weaker and partly retarded development of the cyclone and a weakening of the PV-tower together with reduced diabatic heating rates, particularly at lower and mid levels. We infer that diabatic processes played a crucial role during the phase of rapid deepening of Xynthia and thus to its intensity over the Southeastern North Atlantic. We suggest that windstorms like Xynthia may occur more frequently under future climate conditions due to the warming SSTs and potentially enhanced latent heat release, thus increasing the windstorm risk for Southwestern Europe.

Description: We present a series of idealized numerical model experiments to investigate aspects of deep convection in tropical depressions, including the effects of a boundary layer wind structure on storm structure, especially on vertical vorticity production and updraught splitting, and the combined effects of horizontal and vertical shear on vertical vorticity production, with and without background rotation. In warm-cored disturbances such as tropical depressions, the vertical shear and horizontal vorticity change sign at some level near the top of the boundary layer so that, unlike in the typical middle-latitude ‘supercell’ storm, the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity, but also to a reversal in sign of the updraught rotation with height. This finding has implications for understanding the merger of convectively-induced vorticity anomalies during vortex evolution. Ambient cyclonic horizontal shear and/or cyclonic vertical vorticity favour amplification of the cyclonically-rotating gyre of the dipole. Consistent with an earlier study, storm splitting occurs in environments with pure horizontal shear as well as pure vertical shear, but the morphology of splitting is different. In both situations, splitting is found to require a relatively unstable sounding and relatively strong wind shear.

Description: [1]  We performed shock compression experiments on preheated forsterite liquid (Mg 2 SiO 4 ) at an initial temperature of 2273 K and have revised the equation of state (EOS) that was previously determined by shock melting of initially solid Mg 2 SiO 4 (300 K). The linear Hugoniot, U S  = 2.674 ± 0.188 + 1.64 ± 0.06 u p km/s, constrains the bulk sound speed within a temperature and composition space as yet unexplored by 1-bar ultrasonic experiments. We have also revised the EOS for enstatite liquid (MgSiO 3 ) to exclude experiments that may have been only partially melted upon shock compression and also the EOS for anorthite liquid, which now excludes potentially un-relaxed experiments at low pressure. The revised fits and the previously determined EOS of fayalite and diopside were used to produce isentropes in the multicomponent CaO-MgO-Al 2 O 3 -SiO 2 -FeO system at elevated temperatures and pressures. Our results are similar to those previously presented for peridotite and simplified “chondrite” liquids such that regardless of where crystallization first occurs, the liquidus solid sinks upon formation. This process is not conducive to the formation of a basal magma ocean. We also examined the chemical and physical plausibility of the partial melt hypothesis to explain the occurrence and characteristics of ultralow velocity zones. We determined that the ambient mantle cannot produce an equilibrium partial melt and residue that is sufficiently dense to be a ULVZ mush. The partial melt would need to be segregated from its equilibrium residue and combined with a denser solid component to achieve a sufficiently large aggregate density.

Description: [1]  High resolution sparker and crustal-scale airgun seismic reflection data, coupled with repeat bathymetric surveys, document a region of repeated coseismic uplift on the portion of the Alaska subduction zone that ruptured in 1964. This area defines the western limit of Prince William Sound. Differencing of vintage and modern bathymetric surveys shows that the region of greatest uplift related to the 1964 Great Alaska earthquake was focused along a series of sub-parallel faults beneath Prince William Sound and the adjacent Gulf of Alaska shelf. Bathymetric differencing indicates that 12 m of coseismic uplift occurred along two faults that reached the sea floor as submarine terraces on the Cape Cleare bank southwest of Montague Island. Sparker seismic reflection data provide cumulative Holocene slip estimates as high as 9 mm/yr along a series of splay thrust faults within both the inner wedge and transition zone of the accretionary prism. Crustal seismic data show that these megathrust splay faults root separately into the subduction zone décollement. Splay fault divergence from this megathrust correlates with changes in mid-crustal seismic velocity and magnetic susceptibility values, best explained by duplexing of the subducted Yakutat terrane rocks above Pacific plate rocks along the trailing edge of the Yakutat terrane. Although each splay fault is capable of independent motion, we conclude that the identified splay faults rupture in a similar pattern during successive megathrust earthquakes and that the region of greatest seismic coupling has remained consistent throughout the Holocene.

Description: [1]  We perform a time-lapse analysis of Rayleigh and Love wave anisotropy above an underground gas storage facility in the Paris Basin. The data were acquired with a three-component seismic array deployed during several days in April and November 2010. Phase velocity and back azimuth of Rayleigh and Love waves are measured in the frequency range 0.2-1.1 Hz using a three-component beamforming algorithm. In both snapshots, higher surface wave modes start dominating the signal above 0.4 Hz with a concurrent increase in back azimuth ranges. We fit anisotropy parameters to the array detections above 0.4 Hz using a bootstrap approach which also provides estimation uncertainty and enables significance testing. The isotropic phase velocity dispersion for Love and Rayleigh waves match for both snapshots. We also observe a stable fast direction of NNW-SSE for Love and Rayleigh waves which is aligned with the preferred orientation of known shallow (〈300 m) and deeper (~1000 m) fault systems in the area, as well as the maximum horizontal stress orientation. At lower frequencies corresponding to deeper parts of the basin, the anisotropic parameters exhibit higher magnitude in the November data. This may perhaps be caused by the higher pore-pressure changes in the gas reservoir in that depth range.

Description: [1]  Eruptive activity at the summit of Kilauea Volcano, Hawaii beginning in 2010 and continuing to the present time is characterized by transient outgassing bursts accompanied by very long period (VLP) seismic signals triggered by rockfalls from the vent walls impacting a lava lake in a pit within the Halemaumau pit crater. We use raw data recorded with a 11-station broadband network to model the source mechanism of signals accompanying two large rockfalls on August 29, 2012 and two smaller average rockfalls obtained by stacking over all events with similar waveforms to improve the signal-to-noise ratio. To determine the source centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous medium that takes topography into account. We apply a new waveform inversion method that accounts for the contributions from both translation and tilt in horizontal seismograms through the use of Green's functions representing the seismometer response to translation and tilt ground motions. This method enables a robust description of the source mechanism over the period range 1-1000 s. The VLP signals associated with the rockfalls originate in a source region ~1 km below the eastern perimeter of the Halemaumau pit crater. The observed waveforms are well explained by a simple volumetric source with geometry composed of two intersecting cracks including an east striking crack (dike) dipping 80 ∘ to the north, intersecting a north striking crack (another dike) dipping 65 ∘ to the east. Each rockfall is marked by a similar step-like inflation trailed by decaying oscillations of the volumetric source, attributed to the efficient coupling at the source centroid location of the pressure and momentum changes induced by the rock mass impacting the top of the lava column. Assuming a simple lumped parameter representation of the shallow magmatic system, the observed pressure and volume variations can be modeled with the following attributes: rockfall volume (200 − 4500 m 3 ), length of magma column (120-210 m), diameter of pipe connecting the Halemaumau pit crater to the subjacent dike system (6 m), average thickness of the two underlying dikes (3 – 6 m), and effective magma viscosity (30–210 Pa s). Most rockfalls occur during episodes of sustained deflation of the Kilauea summit. The mass loss rate in the shallow magmatic system is estimated to be 1400  −  15, 000 kg s − 1 based on measurements of the temporal variation of VLP period in the two large rockfalls that occurred on August 29, 2012.

Description: The Northwestern Mediterranean Sea (NWMS) is one of the most productive areas of the Mediterranean Sea. The NWMS pelagic planktonic ecosystem is strongly influenced by hydrodynamics, in particular winter deep convection. Here we investigate the response of this ecosystem and associated carbon cycle to oceanic and atmospheric winter conditions interannual variability. For that we developed a tridimensional coupled physical-biogeochemical model, ran annual simulations forced by XXth climate conditions and performed statistical and budget analysis. Our coupled model reproduces correctly the seasonal evolution of the NWMS pelagic planktonic ecosystem. It however overestimates the contribution of nanophytoplankton to the total phytoplanktonic biomass and GPP, underestimates the bacteria biomass and represents the spring bloom with one month delay. Our results confirm that the control of phytoplanktonic development and bacteria growth by the phosphorus availability is a marked specificity of the NWMS, that is temporally reduced by deep convection. They confirm the relevance of the Behrenfeld [2010] hypothesis in explaining the bloom dynamics. The variability of the winter atmospheric conditions induces differences of vertical mixing and water temperature that propagate into the whole NWMS ecosystem through a chain of relationships. The high frequency filtering associated with averaging diagnostics explains that this variability seems weak at the NWMS scale. However for most of the variables and processes, differences induced by the winter atmospheric variability are significant at the annual scale. Net metabolism and deep carbon export are systematically positive and show larger variabilities related respectively to the water temperature and convection intensity.

Description: Frontal meanderings are generally difficult to predict. In this study, we demonstrate through an exercise with the Iceland-Faeroe Front (IFF) that satisfactory predictions may be achieved with the aid of hydrodynamic instability analysis. As discovered earlier on, underlying the IFF meandering is a convective instability in the western boundary region followed by an absolute instability in the interior; correspondingly the disturbance growth reveals a switch of pattern from spatial amplification to temporal amplification. To successfully forecast the meandering, the two instability processes must be faithfully reproduced. This sets stringent constraints for the tunable model parameters, e.g., boundary relaxation, temporal relaxation, eddy diffusivity, etc. By analyzing the instability dispersion properties, these parameters can be rather accurately set, and their respective ranges of sensitivity estimated. It is shown that too much relaxation inhibits the front from varying; on the other hand, too little relaxation may have the model completely skip the spatial growth phase, leading to a meandering way more upstream along the front. Generally speaking, dissipation/diffusion tends to stabilize the simulation, but unrealistically large dissipation/diffusion could trigger a spurious absolute instability, and hence a premature meandering intrusion. The belief that taking in more data will improve the forecast does not need to be true; it depends on whether the model setup admits the two instabilities. This study may help relieve modelers from the laborious and tedious work of parameter tuning; it also provides us criteria to distinguish a physically relevant forecast from numerical artifacts

Description: In this paper we assess the risk of future coastal flooding in the Severn Estuary, examining the contribution from low probability extreme sea level rise scenarios resulting from the possibility of increased rates of ice sheet mass loss in the coming century. A simple asymmetric probability distribution is constructed to include sea level rise scenarios of up to 1.9 m by 2100, based on recent assessments of future sea level rise in the UK. A regular sampling procedure, sampling every 1 mm, is used to increase the boundary water levels associated with a current 1:200 year event to force a two-dimensional hydrodynamic model of coastal inundation to examine the influence of sea level rise on inundation of the Somerset Levels region. From the resulting ensemble of predictions an estimation of risk (conditioned upon the hazard and the probability of occurrence) by 2100 is established. The results indicate that although the likelihood of extreme sea level rise due to rapid ice sheet mass loss is low, the resulting hazard can be large, resulting in a significant (29.7%) increase to the projected risk. These findings clearly demonstrate that uncertainty in future sea level rise, mostly associated with the rate of ice sheet mass loss, is a vital component of coastal flood risk, and therefore, needs to be accounted for by decision makers when considering mitigation policies related to coastal flooding.

Description: The in-situ data in the Deepwater Navigation Channel (DNC), Yangtze River Estuary (YRE), China, in the dry season 2009, shows spring tides associated with greater maximum velocities, more mixing, less stratification, and diffused fluid mud; whereas neap tides are associated with smaller maximum velocities, greater stratification, inhibited mixing, and stratified fluid muds. The balance of salt flux indicates the seaward salt transport is dominated by fluvial flows, and the landward salt transport is generated by compensation flows during spring tides, but shear effects during neap tidal cycles. The balance of suspended sediment flux illustrates the offshore sediment transport is dominated by fluvial flows as well, but the onshore transport is induced by tidal-pumping effects on spring tides, and shear effects on neaps. The suspended sediment transport is strongly affected by the salinity distribution and salinity-gradient-induced stratification in the DNC. The spring-neap asymmetry is generated by the estuarine gravitational circulation during low-flow conditions; while the flood-ebb asymmetric stratification within a tidal cycle is due to the semidiurnal-tidally movement of the salt front.

Description: In recent years the latitudinal position of the Subtropical Front (STF) has emerged as a key parameter in the global climate. A poleward positioned front is thought to allow a greater salt flux from the Indian to the Atlantic Ocean and so drive a stronger Atlantic Meridional Overturning Circulation. Here, the common view that the STF aligns with the zero windstress curl (WSC) is challenged. Based on the STF climatologies of Orsi [1995], Belkin and Gordon [1996], and Graham and De Boer [submitted], and on satellite scatterometry winds, we find that the zero WSC contour lies on average ~10º, ~8º, and ~5º poleward of the front for the three climatologies, respectively. The circulation in the region between the Subtropical gyres and the zero WSC contour is not forced by the WSC but rather by the strong bottom pressure torque that is a result of the interaction of the Antarctic Circumpolar Current with the ocean floor topography. The actual control of the position of the STF is crucially dependent on whether the front is regarded as simply a surface water mass boundary or a dynamical front. For the Agulhas Leakage problem the southern boundary of the so-called Super Gyre may be the most relevant property but this cannot easily be identified in observations.

Description: The M 2 tidal circulation in Algeciras Bay (Strait of Gibraltar) is analyzed using a 3-D, nonlinear, baroclinic, hydrodynamic model, in conjunction with observed data series. Results show the influence of the density-stratification on the vertical structure of the M 2 currents in Algeciras Bay, although its tidal dynamics shows major differences with respect to the Strait of Gibraltar. Whereas the M 2 currents in the Strait present mainly barotropic behaviour, the baroclinic effects prevail in Algeciras Bay. A notable finding is the presence of a tidal M 2 counter-current system between the upper Atlantic and the lower Mediterranean water layers within the Bay, with amplitudes of up to 25 cm s -1 . The interface between the two layers oscillates in anti-phase relation with respect to the free-surface elevation, with amplitudes of almost 20 m. The presence of the submarine Algeciras Canyon was found to be determinant in the three-dimensional structure of tidal currents within the Bay, strengthening the baroclinic tidal regime of currents. This situation has quantitative consequences for the flow exchange processes between Algeciras Bay and the outer Strait, with rates 20 times higher than those obtained when considering only the barotropic behaviour, as well as inflow/outflow lateral recirculation volumes during half a tidal cycle that account for more than 20% of the net accumulated volume. This flow exchange system was found to be affected by the nonlinear interaction processes between the first baroclinic period of resonance of Algeciras Bay and the M 2 tide.

Description: Impacts of submesoscale processes on transport are investigated numerically in an energetic mesoscale flow with an ocean model run at two horizontal resolutions, 1 km and 5 km. The focus is the northwestern Gulf of Mexico, where the Loop Current eddies are surrounded by smaller vortices. By increasing the horizontal resolution, the number and strength of submesoscale eddies and vorticity filaments within the mixed layer increase dramatically, and with them the vertical velocities. Inside the coherent eddies and at their peripheries increased vertical velocities for increasing resolution are associated to near inertial motions and they are not limited to the mixed layer, but are found at all depths. Horizontal velocities, on the contrary, are similar. Lagrangian isobaric tracers are deployed close to the surface and at 100 m, and three dimensional, neutrally buoyant particles are released close to the surface, at the base of the mixed layer and at 100 m. The modeled horizontal dispersion curves for each deployment depth are independent of the kind of particles and of horizontal resolution. Close to the ocean surface, however, convergence zones, generated by submesoscale ageostrophic motions and resolved at 1 km resolution, influence the details of the tracer distributions. Vertical dispersion increases by several folds for increasing resolution at all depths explored, with the largest differences found close to the surface. Therefore submesoscales processes play a fundamental role in driving vertical transport in eddy dominated flows, both within and below the mixed layer, for times comparable to the Eulerian time scale.

Description: The 2011 Tohoku tsunami devastated the northeastern Japan coasts and caused localized damage to coastal infrastructure across the Pacific. The tsunami resulted in strong currents around the Hawaiian Islands that led to closure of harbor and marinas for up to 38 hours after its arrival. We utilize a non-hydrostatic model to reconstruct the tsunami event from the seismic source for elucidation of the physical processes and inference of coastal hazards. A number of tide gauges, bottom pressure sensors, and ADCPs provided point measurements for validation and assessment of the model results in Hawaii. Spectral analysis of the computed surface elevation and current reveals complex flow patterns due to multi-scale resonance. Standing waves with 33 to 75 min period develop along the island chains, while oscillations of 27 min or shorter are primarily confined to an island or an island group with interconnected shelves. Standing edge waves with periods 16 min or shorter, which are able to form nodes on the reefs and inside harbors, are the main driving force of the observed coastal currents. Resonance and constructive interference of the oscillation modes provide an explanation of the impacts observed in Hawaii with implications for emergency management in Pacific island communities.

Description: This study characterizes the seasonal cycle of the Catalan inner-shelf circulation using observations and complementary numerical results. The relation between seasonal circulation and forcing mechanisms is explored through the depth-averaged momentum balance, for the period between May 2010 and April 2011 when velocity observations were partially available. The monthly-mean along-shelf flow is mainly controlled by the along-shelf pressure gradient and by surface and bottom stresses. During summer, fall and winter, the along-shelf momentum balance is dominated by the barotropic pressure gradient and local winds. During spring, both wind stress and pressure gradient act in the same direction and are compensated by bottom stress. In the cross-shelf direction the dominant forces are in geostrophic balance, consistent with dynamic altimetry data.

Description: The Southern Ocean Subtropical Front (STF) is thought to play a key role in the global climate system. Theory suggests that the latitude of the STF regulates the volume of saline Agulhas Leakage into the Atlantic Ocean from the Indian. Here we use satellite sea surface temperature (SST) and height (SSH) data to study the physical characteristics of the STF water mass boundary. We find that the strong currents in this region do not align with the surface water mass boundary. Therefore we provide a new climatology for these currents which we define as the Dynamical STF (DSTF). The DSTF is the eastward extension of the western boundary current in each basin and marks the southern boundary of the subtropical gyre circulations. It is a deep water mass boundary. At the centre of each basin it merges with the Sub-Antarctic Front. The DSTF is characterised by strong SST and SSH gradients, and has no seasonal cycle. On the eastern side of basins the STF surface water mass boundary coincides with a separate region of enhanced temperature gradients which we call the Subtropical Frontal Zone (STFZ). The STFZ is comprised of multiple SST fronts and has a large seasonal cycle. There are no strong SSH gradients in the STFZ. Separating the DSTF and STFZ is a band of weak SST gradients. Given the clear separation of these features as well as stark contrast in characteristics and seasonal cycles it is counterproductive and misleading to refer to them both as the STF.

Description: A time-dependent, 1D coupled ice-ocean model is used to quantify the impact of ocean stratification on the Arctic ice cover. The model results show that the ice growth during winter equals the ice melt in summer for areas with a well-developed cold halocline layer (CHL), provided that the initial ice thickness is around 3 m, while thinner initial ice thickness results in net growth. Areas with weak salt stratification can have a negative annual thickness change irrespective of the initial ice thickness and are thus dependent on ice import in order to remain ice-covered. The model results also show that ocean stratification is mostly important for ice-thickness development during the growing season. Areas with weak stratification have an ocean heat flux up to 8 Wm -2 reaching the ice during the growing season, while areas with a CHL have an average of about 0.7 Wm -2 . In the extreme area north of Svalbard, the ocean heat fluxes are typically around 25 Wm -2 but can be up to 400 Wm -2 during the initial adjustment, when the warm Atlantic water has direct contact with the ice. A general outcome of the study is that, depending on ocean stratification, the ice cover of Arctic Ocean can be divided into one part with net ice growth (the major part) and another part with net ice melt (mainly in the Nansen Basin).

Description: [1]  On 5 September 2012, a large thrust earthquake (M w 7.6) ruptured a densely-instrumented seismic gap on the shallow-dipping plate boundary beneath the Nicoya Peninsula, Costa Rica. Ground motion recordings directly above the rupture zone provide a unique opportunity to study the detailed source process of a large shallow megathrust earthquake using very nearby land observations. Hypocenter relocation using local seismic network data indicates that the event initiated with small emergent seismic waves from a hypocenter ~10 km offshore, 13 km deep on the megathrust. A joint finite-fault inversion using high-rate GPS, strong-motion ground velocity recordings, GPS static offsets, and teleseismic P waves reveals that the primary slip zone (slip 〉 1 m) is located beneath the peninsula. The rupture propagated down-dip from the hypocenter with a rupture velocity of ~3.0 km/s. The primary slip zone extends ~70 km along strike and ~30 km along dip, with an average slip of ~2 m. The associated static stress drop is ~3 MPa. The seismic moment is 3.5 x 10 20  Nm, giving M w  = 7.6. The co-seismic large-slip patch directly overlaps an onshore inter-seismic locked region indicated by geodetic observations, and extends down-dip to the intersection with the upper plate Moho. At deeper depths, below the upper plate Moho, seismic tremor and low frequency earthquakes have been observed. Most tremor locates in adjacent areas of the megathrust that have little co-seismic slip; a region of prior slow slip deformation to the southeast also has no significant co-seismic slip or aftershocks. An offshore locked patch indicated by geodetic observations does not appear to have experienced co-seismic slip, and aftershocks do not overlap this region, allowing the potential for a comparable size rupture offshore in the future.

Description: [1]  Seismic shear waves emitted by earthquakes can be modeled as plane (transverse) waves. When entering an anisotropic medium they can be split into two orthogonal components moving at different speeds. This splitting occurs along an axis, the fast polarization, that is determined by geologic conditions. We present here a comprehensive analysis of the Silver and Chan (1991) method, used to obtain shear wave splitting parameters, comprising theoretical derivations and statistical tests of the assumptions used to construct the standard errors. We find discrepancies in the derivations of equations in their article, with the most important being a mistake in how the standard errors are calculated. Our simulations suggest that the degrees of freedom are being overestimated by this method and consequently the standard errors are too small. Using a set of S waveforms from very similar shallow earthquakes on Reunion Island, we perform a statistical analysis on the noise of these replicates and find that the assumption of Gaussian noise does not hold. Further, the properties of background noise differ substantially from the noise obtained from the shear wave splitting analysis. However, we find that the standard errors for the fast polarization are comparable to the spread in the fast polarization parameters. Delay time errors appear to be comparable to delay time estimates once cycle skipping is accounted for. Future work using synthetic seismograms with simulated noise should be conducted to confirm this is the case for earthquakes in general.

Description: [1]  The sliver strike-slip Great Sumatra Fault (GSF) traverses mainland Sumatra from the Sunda Strait in the southeast to Banda Aceh in the northwest, and defines the present day plate boundary between the Sunda Plate in the north and the Burmese Sliver Plate in the south. It has been well studied on mainland Sumatra but poorly north of Banda Aceh in the Andaman Sea. Here we present deep seismic reflection images along the northward extension of the GSF over 700 km until it joins the Andaman Sea Spreading Centre and we interpret these images in the light of earthquake, gravity, and bathymetry data. We find that the GSF has two strands between Banda Aceh and Nicobar Island: a transpression in the south and a deep narrow active rift system in the north dotted with volcanoes in the center, suggesting that the volcanic arc is coincident with rifting. Further north of Nicobar Island, an active strike-slip fault, the Andaman-Nicobar Fault, cuts through a rifted deep basin until its intersection with the Andaman Sea Spreading Centre. The volcanic arc lies just east of the rift basin. The western margin of this basin seems to be a rifted continental margin, tilted westward, and flooring the Andaman-Nicobar forearc basin. The Andaman-Nicobar forearc basin is bounded in the west by backthrusts similar to the West Andaman and Mentawai faults. The cluster of seismicity after the 2004 great Andaman-Sumatra earthquake just north of Nicobar Island coincides with the intersection of two strike-slip fault systems.

Description: [1]  The Japan Tohoku-Oki earthquake (9.0 Mw) of 11 March 2011 has left signatures in the Earth's gravity field that are detectable by data of the GRACE mission. Because ESA's satellite gravity mission GOCE – launched in 2009 – aims at high spatial resolution, its measurements could complement the GRACE information on coseismic gravity changes, although time-variable gravity was not foreseen as goal of the GOCE mission. We modeled the coseismic earthquake geoid signal and converted this signal to vertical gravity gradients at GOCE satellite altitude. We combined the single gradient observations in a novel way reducing the noise level, required to detect the coseismic gravity change, subtracted a global gravity model, and applied tailored outlier detection to the resulting gradient residuals. Furthermore, the measured gradients were along-track filtered using different gradient bandwidths where in the space domain Gaussian smoothing has been applied. One year periods before and after earthquake occurrence have been compared with the modeled gradients. The comparison reveals that the earthquake signal is well above the accuracy of the vertical gravity gradients at orbital height. Moreover, the obtained signal from GOCE shows a 1.3 times higher amplitude compared with the modeled signal. Besides the statistical significance of the obtained signal, it has a high spatial correlation of ~0.7 with the forward modeled signal. We conclude therefore that the coseismic gravity change of the Japan Tohoku-Oki earthquake left a statistically significant signal in the GOCE measured gravity gradients.

Description: [1]  The Ninetyeast Ridge (NER), one of the longest linear volcanic features on the Earth, offers an excellent opportunity of understanding the isostatic response to the interactions of mantle plume with the migrating mid-ocean ridge. Bathymetry, geoid and gravity (ship-borne and satellite) data along 72 closely spaced transects and 17 overlapping grids on the NER are analyzed and modeled to determine the effective elastic thickness ( Te ) beneath the entire ridge. The results of 2-D and 3-D flexural modeling of the NER show large spatial variations in Te values ranging from 4 to 35 km, suggesting that the ridge was compensated along its length by different isostatic mechanisms. The southern (south of 22°S latitude) and northern (north of 2°N latitude) parts of the NER have Te values of 〉10 and 〉23 km, respectively, revealing that the southern part was emplaced on a lithosphere of intermediate strength possibly on flank of the Indian plate, whereas the northern part was emplaced in an intraplate setting. In contrast, in the central part of the NER (between latitudes 22°S and 2°N), highly variable Te values (4–22 km) are estimated. The scattered Te values in the central NER suggest that this part may have evolved due to the occurrence of more frequent ridge jumps caused by the interaction of Kerguelen hot spot with rapid northward migration of the Wharton spreading ridge. Residual Mantle Bouguer Anomaly (RMBA) map of the NER and adjacent basins reveals that the entire length of the NER is associated with a significant negative anomaly up to 200 mGal, indicating the presence of thickened crust or less dense mantle beneath the ridge. 3-D crustal thickness map of the NER, generated by inversion of the RMBA data, shows a thick crust ranging from 15 to 19 km. The present study clearly shows that NER possesses a highly segmented isostatic pattern with the occurrence of sub-crustal underplating or sub-surface loading.

Description: : A three-dimensional wave-current-sediment coupled numerical model with wetting and drying process is developed to understand hydrodynamics and sediment transport dynamics in the Deepwater Navigation Channel (DNC), the North Passage of the Yangtze River Estuary (YRE), China. The model results are in good agreement with observed data, and statistics show good model skill scores and correlation coefficients. The model well reproduces the spring-neap variation between a well-mixed estuary and a highly-stratified estuary. Model results indicate that the estuarine gravitational circulation plays the most important role in the estuarine turbidity maximum (ETM) formation in the DNC. The upstream non-local sediment intrusion through the spill-over-mechanism is a major source of sediment trapping in the North Passage after the morphological changes. Numerical studies are conducted to show scenarios in the YRE under the effects of different forcings (river-discharges, waves, and winds). Between these study cases, surface-wave-breaking relieves the sediment trapping and bottom-wave-current-interaction aggravates the bed erosion and elevates the SSC in the ETM; the former and the latter have the least and largest influence on the suspended sediment transport in the DNC. The wind effects have a greater influence on sediment trapping than the river-discharges, and the steady-northwesterly-wind condition favours the siltation in the DNC most. The significance of density-driven turbidity current is also assessed, which can enhance the saline-water intrusion and suppress the turbulent mixing in the bottom boundary layer.

Description: [1]  We conducted deep-sea magnetic measurements using autonomous underwater vehicles in the Bayonnaise knoll caldera, the Izu-Ogasawara island arc, which hosts the large Hakurei hydrothermal field. We improved the conventional correction method applied for removing the effect of vehicle magnetization, thus greatly enhancing the precision of the resulting vector anomalies. The magnetization distribution obtained from the vector anomaly data shows an ∼ 2-km-wide belt of high magnetization, trending NNW–SSE going through the caldera, and a low magnetization zone ∼ 300 m by ∼ 500 m in area, extending over the Hakurei site. Comparison between the results obtained using the vector anomaly and the total intensity anomaly shows that the magnetic field is determined more accurately, especially in areas of sparse data distribution, when the vector anomaly rather than the total intensity anomaly is used. We suggest a geologically motivated model that basaltic volcanism associated with the backarc rifting occurred after the formation of the caldera, resulting in the formation of the high magnetization belt underneath the silicic caldera. The Hakurei hydrothermal field lies in the intersection of the basaltic volcanism belt and the caldera wall fault, suggesting a mechanism that hot water generated by the heat of the volcanic activity has been spouting out through the caldera wall fault. The deposit apparently extends beyond the low magnetization zone, climbing up the caldera wall. This may indicate that hot water rising from the deep through the alteration zone is transported laterally when it comes near the seafloor along fissures and fractures in the caldera wall.

Description: [1]  Using multiple ScS reverberations we examine mantle reflectivity structure beneath northeast China and the northwest Pacific. We find several upper mantle discontinuities, including a melt layer with a mean thickness of 64 km atop the 410-km discontinuity, present on both sides of the subducting slab near the Nankai trench. The transition zone contains a split 520-km discontinuity in several paths, and tomographic images show stagnant slabs at this depth. We believe this may be slab-related based on experimental work (Saikia, A., Frost, D. J., Rubie, D. C., 2008. Splitting of the 520-kilometer seismic discontinuity and chemical heterogeneity in the mantle. Science 319 (5869), 1515–1518). A negative reflector is found in one path beneath the northeast China craton at a depth of 598 km. Mid-mantle reflectors are found in all of our paths and are present throughout a wide depth range (~750 – 1600 km).

Description: [1]  Three-dimensional P - and S -wave velocity (V P , V S ) models and high-resolution earthquake relocations are determined for the New Madrid Seismic Zone using double-difference local earthquake tomography. The data set consists of arrival times and differential times recorded by the Cooperative New Madrid Seismic Network (CNMSN) from 2000-2007 and the 1989-1992 Portable Array Network and Data Acquisition deployment. Waveform cross-correlation derived differential times for the CNMSN data are also incorporated. The velocity solutions are compatible with previous solutions centered on the active arms of seismicity and cover a broader area including mafic intrusions along the margin of the Reelfoot Rift. Major features include elevated V P and V S associated with the mafic plutons and reduced V P and V S along and southeast of the Axial fault (AF), a major arm of seismicity trending along the rift axis. Low V P extends to a depth of at least 20 km along the portion of the AF that extends south of the Missouri bootheel. A locally high V P /V S anomaly imaged along the central portion of the Reelfoot fault is spatially correlated with a significant change in fault trend and is interpreted as a region containing high pore pressure and/or water-filled microcracks.

Description: [1]  We present a catalog of InSAR constraints on deformation that occurred during earthquake sequences in southern Iran between 1992-2011, and explore the implications on the accommodation of large-scale continental convergence between Saudi Arabia and Eurasia within the Zagros Mountains. The Zagros Mountains, a salt-laden fold-and-thrust-belt involving ~10 km of sedimentary rocks overlying Precambrian basement rocks, have formed as a result of ongoing continental collision since 10-20 Ma that is currently occurring at a rate of ~3 cm/yr. We first demonstrate that there is a biased misfit in earthquake locations in global catalogs that likely results from neglect of 3D velocity structure. Previous work involving two M ~ 6 earthquakes with well-recorded aftershocks has shown that the deformation observed with InSAR may represent triggered slip on faults much shallower than the primary earthquake, which likely occurred within the basement rocks (〉10 km depth). We explore the hypothesis that most of the deformation observed with InSAR spanning earthquake sequences is also due to shallow, triggered slip above a deeper earthquake, effectively doubling the moment release for each event. We quantify the effects that this extra moment release would have on the discrepancy between seismically and geodetically constrained moment rates in the region, finding that even with the extra triggered fault slip, significant aseismic deformation during the interseismic period is necessary to fully explain the convergence between Eurasia and Saudi Arabia.

Description: The Arctic Ocean is changing rapidly but there are no longterm time series observations on the state of the phytoplankton community that could allow a link to be made from physical/chemical pressures to the impact on marine ecosystems. Here, we test the idea that space-for-time (SFT) substitution might predict temporal change in the Canada Basin premised on differences in the present state of phytoplankton in other geographic zones, specifically the ratio in the abundance of picophytoplankton to nanophytoplankton (Pico:Nano). We compared the change in Pico:Nano observed in the Canada Basin from 2004 to 2012 to the different average states of this ratio in 26 other ocean ecological regions. Our results show that as upper ocean nitrate concentration changed in the Canada Basin from year to year, the concomitant change in Pico:Nano was statistically commensurate with the difference that this ratio exhibits between Longhurst ecological provinces in relation to nitrate concentration. Lower average concentration of nitrate in the upper water column is associated with a higher value of Pico:Nano, a result consistent with resource control of phytoplankton size structure in the ocean. We suggest that SFT substitution allows an explanation of temporal progression from spatial pattern as a test of mechanism, but such statistical prediction is not necessarily a projection of future states.

Description: [1]  The identification and evaluation of trigger mechanisms for volcano flank instabilities and/or collapse represent a key issue for risk assessment in densely populated volcanic areas, as well as in long distance settings, particularly in case of island or coastal volcanoes. Here, we address quantitatively the effects of external (seismic) and inner (magmatic) forcing on the stress-strain state associated to flank instabilities at Mt. Etna (Sicily, southern Italy) by means of a 2-D Finite-Difference-Method numerical modelling. Modelled seismic actions include strong near-field, strong far-field and low-magnitude near-field earthquakes. Magmatic actions consider the inner presssure changes induced by energetic lava fountains in the summit crater area, sub-vertical and oblique dyke ascent below the summit area. Model results are validated in light of available monitoring data and recent eruptive activity. Numerical results show that the main strain effects are produced by high-magnitude near-field earthquakes (expected return time of ~10 3  yrs), and by vertical uprise of a magma dyke below the volcano summit area. Maximum displacements in the order of tens of centimetres may involve the summit area, up to some 10 6  m 3 /m over some km laterally. Stress releases up to 10 7  Pa may affect a limited portion of the magmatic conduit, thus favouring major effusive flank eruptions. Major catastrophic events, such as volcano flank collapse, should not be expected by applying, either individually or combined, the aforementioned actions.

Description: [1]  Seismicity closely related to hydrological impacts has been observed in several locations worldwide; particularly in intraplate areas where tectonic stressing rates are small. The triggering mechanism is usually explained by a poroelastic response of the seismogenic crust to surface water flux, leading to pore pressure changes at depth. To explain the earthquake triggering in response of those small stress changes, however, the crust has to be near a critical state in which other transient processes might be significant. One of the prominent examples is the Mt. Hochstaufen in SW Germany, where seismicity is known to vary seasonally. A previous analysis showed that the seismicity in 2002 was highly correlated with model forecasts based on fluid diffusion and rate- and state-dependent frictional nucleation. Here we revisit this case by accounting additionally for poroelastic effects, as well as for thermoelastic and tidal stresses. We also test whether the model can explain the observations of the subsequent eight years between 2003 and 2010. Our analysis confirms that rainfall is the dominant driving force in this region. The model not only fits the year 2002 activity very well, but provides with the same parameters a reasonable fit to the subsequent period, with a probability gain of about 4 per event in comparison to a time-independent Poisson model.

Description: ABSTRACT [1]  The absolute magnitude of stress in the crust and the shear strength of faults are poorly known, yet fundamental quantities, in lithospheric dynamics. While stress magnitude cannot be measured directly, deviatoric stress state can be inferred indirectly from focal mechanism solutions collected before and after an earthquake. We extend a standard stress inversion for normalized stresses to invert for the 3D spatial distribution of absolute deviatoric stress and variation of fault strength with depth using focal mechanism solutions and coseismic stress changes produced by large earthquakes. We apply the method to the 2011 M9 Tohoku-oki, Japan earthquake. The northern Japan forearc crust between 5 and 15 km depth appears to be weak with fault strength of 40–90 MPa, consistent with a coefficient of friction of 0.2-0.5. The M9 Tohoku-oki coseismic stress change was large enough, relative to the ambient stress, to rotate the principal stress directions typically ~20° in the upper 20 km of the crust. The data from Japan require a heterogeneous ambient deviatoric stress field with short wavelength (~20-50 km) fluctions in principal stress orientations.

Description: Particle aggregation plays an important role in many marine biogeochemical processes such as determining the vertical flux of particulate material and trace metal scavenging. Models of particle aggregation vary in complexity and in this paper I compare the behavior of a detailed size-spectrum model with that of a series of simple, two size-class models with different representations of aggregation, all of which have appeared in the literature. The simplest model uses a first-order representation of aggregation kinetics, while two other models have non-linear representations of aggregation. The simplest model is unable to reproduce the dynamic or steady-state behavior of the size-spectrum model. Results from the two non-linear size-class models show better agreement with the behavior of the size-spectrum model. I find that the mode of aggregation and the dependence of aggregation on particle size are crucial for understanding the differences between the models.

Description: This study investigates how the quality of sea surface temperature (SST) observations made by drifting buoys (drifters) and ships for 1996-2010 can be improved through retrospective quality control (QC) against a reference field. The observations used are a blend of delayed mode data taken from the International Comprehensive Ocean-Atmosphere Data Set (versions 2.0 and 2.5) and real time data obtained from the Global Telecommunication System. A comparison of drifter and ship measurements on a platform-by-platform basis to high quality SST estimates from the ATSR Reprocessing for Climate (ARC) project reveals drifter observations are generally of good quality but frequently suffer from gross errors, whilst ship observations are generally of worse quality and show a diverse range of measurement errors. QC procedures are developed which similarly assess drifter and ship SST observations through comparison with the Met Office Operational SST and Sea Ice Analysis (OSTIA). These procedures make use of seasonal background error variance estimates now available for OSTIA. Drifter observations displaying some commonly observed gross errors are flagged and ship callsigns whose observations are deemed unreliable are blacklisted. Validation of the QC outcomes against ARC and Argo demonstrates that this retrospective QC improves the quality of drifter and ship observations, though some limitations are discussed.

Description: Observations of the spatially dependent velocity field over movable bedforms subjected to slightly skewed and asymmetric regular wave forcing were collected. The dynamics between the ripple elements is dominated by coherent vortices, characterized by the swirling strength, and evidenced in the temporal and spectral characterization. Within the boundary layer, spectral energy in the second harmonic (3 f 0 ) is amplified at the ripple slopes and is consistent with the location of the expected strongest pressure gradients. First- and second-moment velocity statistics were used to address the spatial variability of the intra-ripple hydrodynamics. Estimates of displacement and momentum thicknesses (δ* and δ mom ), are smaller than suggested by the higher harmonics, but consistently highlight areas of adverse and favorable pressure gradients. Shear stress and roughness estimates were inferred by fitting a logarithmic model to first- and second-moment statistics of the velocity field. The maximum Shields parameter was observed to peak at the stoss slope of asymmetric ripples during the strongest and shorter half-wave period (onshore). First-moment roughness estimates are similar in magnitude to bedload parameterizations provided by Li et al . [1997], and about a factor of 3 larger than second-moment estimates. Assessment of the vertical transfer of horizontal momentum derived using a Reynolds decomposition, suggests that stresses inferred from the logarithmic law using first-moment velocity statistics appropriately reproduce the mean momentum transfer for the longer and weaker offshore half-wave period.

Description: The tidal regime in the Bohai Sea, China, is investigated using observations and an established numerical tidal model. The area has recently experienced rapid coastline changes due to natural developments of the Yellow River delta and large-scale anthropogenic land reclamation. These morphological changes are not reflected in most global bathymetric databases and are thus rarely incorporated into investigations of the Bohai Sea. It is shown that there have indeed been significant changes in the tidal regime in the Bohai Sea over the last 35 years, with M 2 amplitudes changing up to 20 cm in some parts. The model captures some of these changes when the appropriate bathymetries are used. Furthermore, the simulations show that the tides in the Bohai Sea have become more sensitive to future sea level rise and the way in which it is implemented in the model (i.e. whether or not flood defenses are included).These sensitivity changes are due to the recent coastal developments.

Description: We investigate the temporal variability and trends of pH and of the aragonite saturation state, Ω arag , in the southern California Current System on the basis of a 6 year timeseries from Santa Monica Bay, using bi-weekly observations of dissolved inorganic carbon and combined calculated and measured alkalinity. Median values of pH and Ω arag in the upper 20 m are comparable to observations from the subtropical gyres, but the temporal variability is at least a factor of 5 larger, primarily driven by short-term upwelling events and mesoscale processes. Ω arag and pH decrease rapidly with depth, such that the saturation horizon is reached already at 130 m, on average, but it occasionally shoals to as low as 30 m. No statistically significant linear trends emerge in the upper 100 m, but Ω arag and pH decrease, on average, at rates of -0.009 ± 0.006 yr -1 and -0.004 ± 0.003 yr -1 in the 100 to 250 m depth range. These are somewhat larger, but not statistically different from the expected trends based on the recent increase in atmospheric CO 2 . About half of the variability in the deseasonalized data can be explained by the El Niño Southern Oscillation (ENSO), with warm phases (El Niño) being associated with above normal pH and Ω arag . The observed variability and trend in Ω arag and pH is well captured by a multiple linear regression model on the basis of a small number of readily observable independent variables. This permits the estimation of these variables for related sites in the region.

Description: [1]  We review marine heat flow data along the Nankai Trough and show that observations 〉 30 km seaward of the deformation front are 20% below conductive predictions (129–94 mW m -2 ) but consistent with the global heat flow average for oceanic crust of the same age (16-28 Ma). Heat flow values  〈  30 km seaward of the deformation front are generally 20% higher than conductive predictions. This heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench, 〉 200 and 〉 140 mW m -2 , and steep landward declines to values of approximately 60 mW m -2 over distances of 65 and 50 km along the Muroto and Kumano transects, respectively. Along the Ashizuri transect the lack of heat flow data preclude a definitive interpretation. We conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25 ° C higher near the toe of the margin wedge and 50 - 100 ° C lower near the downdip limit of the seismogenic zone than estimated by purely conductive models.

Description: Observations have been obtained within an intense (precipitation rates 〉 50 mm hour -1 ) narrow cold-frontal rainband (NCFR) embedded within a broader region of stratiform precipitation. In-situ data were obtained from an aircraft which flew near a steerable dual-polarisation Doppler radar. The observations were obtained to characterise the microphysical properties of cold frontal clouds, with an emphasis on ice and precipitation formation and development. Primary ice nucleation near cloud top (−55°C) appeared to be enhanced by convective features. However, ice multiplication led to the largest ice particle number concentrations being observed at relatively high temperatures (〉 −10°C). The multiplication process (most likely rime-splintering) occurs when stratiform precipitation interacts with supercooled water generated in the NCFR. Graupel was notably absent in the data obtained. Ice multiplication processes are known to have a strong impact in glaciating isolated convective clouds, but have rarely been studied within larger organised convective systems such as NCFRs. Secondary ice particles will impact on precipitation formation and cloud dynamics due to their relatively small size and high number density. Further modelling studies are required to quantify the effects of rime splintering on precipitation and dynamics in frontal rainbands. Available parameterizations used to diagnose the particle size distributions do not account for the influence of ice multiplication. This deficiency in parameterizations is likely to be important in some cases for modelling the evolution of cloud systems and the precipitation formation. Ice multiplication has significant impact on artefact removal from in-situ particle imaging probes.

Description: This paper proposes a selective ensemble mean technique for tropical cyclone (TC) track forecast based on the errors of ensemble prediction system (EPS) members at short lead times (SLTs, 12 h in this study). The means (SEAV) and weighted means (SEWE) of selected EPS members are applied to EPS products from the European Centre for Medium-Range Weather Forecasts (ECMWF), Japan Meteorological Agency (JMA), National Centers for Environmental Prediction (NCEP), and China Meteorological Administration for 35 TCs in the Western North Pacific in 2010 and 2011. Verification results show that SEAV behaves better than SEWE, with a skill of 5% to 30% over relevant ensemble means of EPS within 72 h. The SEAV method is the most effective for the JMA EPS, with a skill of 10% even at 96 h. SEAV predictions are compared with the high-resolution deterministic model predictions of ECMWF and several official forecasts, with special consideration given to the time delay associated with numerical model products in operation. The SEAV for the ECMWF EPS can overcome the high-resolution ECMWF deterministic model at 24 h. Case analyses and sensitivity tests on the error thresholds of member selection and SLT lead times are also presented in this paper.

Description: The predictive quality of an ensemble model of cirrus ice crystals to model passive and active measurements of ice cloud, from the ultraviolet (UV) to the microwave, is tested. The ensemble model predicts ice mass ∝ D 2 (m-D), where D is the maximum dimension of the ice crystal, and m is the mass. This predicted m-D relationship is applied to a moment estimation parametrization of the particle size distribution (PSD), to estimate the PSD shape, given ice water content (IWC) and in-cloud temperature. The same microphysics is applied across the electromagnetic spectrum to model UV, infrared, microwave and radar observations. The short-wave measurements consist of airborne UV backscatter lidar estimates of the volume extinction coefficient, total solar optical depth, and space-based multi-directional spherical albedo measurements, at 0.865 µm, between the scattering angles 85 o and 125 o . The airborne long-wave measurements consist of high-resolution interferometer upwelling brightness temperatures, obtained between the wavelengths of about 3.45 µm and 4.1 µm, and 8.0 µm to 12.0 µm. The low frequency measurements consist of ground-based Chilbolton 35 GHz radar reflectivity measurements and space-based upwelling 190 GHz brightness temperature measurements. The predictive quality of the ensemble model is demonstrated to be generally within the experimental uncertainty of the lidar backscatter estimates of the volume extinction coefficient and total solar optical depth. The ensemble model prediction of the high-resolution brightness temperature measurements is generally within ±2 K and ±1K, at solar and infrared wavelengths, respectively. The 35 GHz radar reflectivity and 190 GHz brightness temperatures are generally simulated to within ±2 dBZ e , and ±2 K, respectively. The directional spherical albedo observations suggest that the scattering phase function of the most randomized ensemble model gives the best fit to the measurements (generally within ±3%). This paper demonstrates that the ensemble model, assuming the same microphysics , is physically consistent across the electromagnetic spectrum.

Description: This paper investigates two schemes that perturb sea-surface temperatures (SSTs) and soil moisture content (SMC) in the Met Office Global and Regional Ensemble Prediction System (MOGREPS), to address a known deficiency of a lack of ensemble spread near the surface. Results from a two-month long trial during the northern hemisphere summer show positive benefits from these schemes. These include a decrease in the spread deficit of surface temperature and improved probabilistic verification scores. SST perturbations exhibit a stronger impact than SMC perturbations, but when combined the increased spread from the two schemes is cumulative. A regional ensemble system driven by the global ensemble members largely reflects the same changes seen in the global ensemble but cycling fields, like SMC, between successive regional forecasts does show some benefit.

Description: [1]  We investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P-wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past ~45 Myr. The imaged Lesser-Antilles slab consists of a northern and southern anomaly, separated by a low velocity anomaly across most of the upper mantle, which we interpret as the subducted North America-South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheatre-shaped slab and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northeastern boundary of the Caribbean plate. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos-Nazca spreading center. In the lower mantle two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an ‘Intra-Americas’ origin and a ‘Pacific origin’ of the Caribbean plate.